Complex Alloy Electroplating Method

ABSTRACT

One embodiment is for the close agitation of electroplating solution substantially near an electrode in an electroplating cell. The agitation occurs within the Nernst diffusion layer allowing for ion replenishment of the electroplating solution at the working electrode face. The system operates by producing a flat agitation face ( 180 ) and a working electrode ( 350 ) face that are brought to within 10 microns of one another during the plating process whilst the agitator  380  is being actuated. In one embodiment, the working electrode is rigid such as a semiconductor wafer. In another embodiment, the working electrode is a flexible material ( 600 ) such as fabric or flexible electronics. Other embodiments are described and shown.

FIELD OF THE INVENTION

This invention covers the field of electroplating and electroplater design.

BACKGROUND OF THE INVENTION

A problem faced when electroplating alloys is that the various elements in the solution deplete at different rates due to different deposition rates of the various elements on to the working electrode. This, in turn, is due to the kinetics of the working electrode reactions and can be partially compensated for by changing the ratio of elements in the solution. However, this ratio needs to be maintained right in the area of close physical proximity to the working electrode, and not just averaged over the entire solution reservoir. This problem is only accentuated for specific applications which require the stoichiometry of the alloys to be held within a narrow band. To increase the availability of ‘fresh’ solution at the working electrode, different methods of agitating the electroplating solution have been developed. These date back to 1917 when Gilber et al produced the first agitated electroplater. This method could be used with limited results for depositing single metals, such as copper, but was inadequate for alloys. It also tended to “streak” the electroplated element(s) where the most agitation was occurring. This made it inadequate for applications where an even deposition is required, such as semiconductor wafer manufacture, flexible electronics or shielding on cloth.

Other designs were also proposed: actual agitation at the working electrode (the cathode) by Pianowki et al; continuous agitation flow by Chen et al, and Pellegrion et al. Alternate methods of agitation by elaborate jet systems were proposed by Keigler et al that use CFD (Computational Fluid Dynamics) to channel a flow of solution over the electroplating surface. These methods all concerned overall solution flow during electroplating. Although streaking was lessened to varying degrees in these methods, they still failed to address the problem of ion depletion in the area in close physical proximity to the working electrode—the Nernst diffusion layer. This boundary layer exists around an electrode in a solution, and is approximately 200 microns thick. Without accounting for the Nernst diffusion layer, ion depletion will continue to be an issue, even with ultrasonic agitation methods; leading to variance in composition of the deposition dependent on the deposition thickness.

Castellani et al even built an ‘aggressive’ agitation system to deposit Permalloy for IBM. This system used an isolated anode and cathode, an argon blanket to stop oxidation of the solution in air, and additional filtering and heat control. However, this system still did not address ion replenishment in the Nernst diffusion layer and as such deposition of the Permalloy's stoichiometry could still vary.

Cao et al drove a fixed current into the cell and measured the voltage in order to monitor the kinematics of the electroplating cell. While this method gave broad feedback on the rate of deposition for single elemental deposited species, it was insufficient to determine stoichiometry to the degree required by complex alloys. It also did not address ion replenishment in the Nernst diffusion layer.

SUMMARY OF THE INVENTION

The invention provides a system for the electroplating of complex alloys with consistent stoichiometry across the complete electroplating surface.

In one embodiment of the invention, the system has a ridged and flat (undulation and bow below 1 um (micrometer)) chuck that holds a semiconducting or MEMS wafer. It includes an agitator that has blades machined such that their edges form a reference plane held to a flatness better than 1 um. This agitator runs on rails that are also engineered to be flat and parallel to within 1 um. Hence, the flatness of the plane on which the agitation blades ride during operation is under a maximum of 2 um. By using micrometer control in conjunction with feedback it is possible to offer the working electrode (the wafer, in this embodiment) up to the agitator within 5 um of the plane of agitation; well within the Nernst diffusion layer. Any other combination of the agitator, blades, rails, and working electrode flatnesses that allows agitation within 200 microns from the working electrode is also within the scope of the invention.

In another embodiment of the invention the chuck is modified so that instead of holding a semiconducting wafer it holds a conducting fabric or flexible electronics material that passes over the chuck such that a reel-to-reel system of electrodeposition is possible.

In accordance with one aspect of the invention, the anode chamber is isolated from the main plating chamber via a frit.

In accordance with another aspect of the invention, electrodes on the agitator are used to position the chuck in relation to the agitator.

In accordance with another aspect of the invention, two reservoirs are provided to hold plating solution for the plating chamber and anode solution for the anode chamber. These chambers are connected and solutions can be pumped as required to keep them fresh and full.

In accordance with another aspect of the invention, all chambers of the system have either an Argon blanket or pressurized inert gas protecting the solutions from ambient air and oxidization. In accordance with another aspect of the invention, heating and cooling elements can be used to maintain the solution at a pre-determined temperature.

In accordance with another aspect of the invention, either the deposition material or the working electrode can be superconducting.

Other goals and advantages of the invention will be further appreciated and understood when considered in conjunction with the following description and accompanying drawings. While the following description may contain specific details describing particular embodiments of the invention, this should not be construed as limitations to the scope of the invention but rather as an exemplification of preferable embodiments. For each aspect of the invention, many variations are possible as suggested herein that are known to those of ordinary skill in the art. A variety of changes and modifications can be made within the scope of the invention without departing from the spirit thereof.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference.

DETAILED DESCRIPTION OF THE DRAWINGS

While preferable embodiments of the invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention.

The invention provides methods and devices for electroplating complex alloys using an electroplating method that aligns an agitator operating on a flat plane with an equally flat and parallel working electrode. The electroplater may be used in any application, including but not limited to, semiconductor wafers, superconductors, MEMS fabrication and deposition on fabrics. For example the electroplater could be used to deposit Permalloy metal onto the surface of a silicon wafer in photolithographically pre-defined molds^(†). This would produce a material that is homogenous across the wafer due to the high degree of agitation across the same area. Thus, magnetic transducers processed using this electroplater would likewise be homogenous. The working electrode can be any conducting material; the only stipulation is that it is flat or mounted onto a chuck that is flat so that it can be aligned parallel to the agitator.

FIG. 1 shows the electrode that is used to align the agitator to the working electrode. This consists of an inert electrode such as platinum 100 that is encased in a polymer 110. The front face of this electrode 120 is machined flat to have undulations less than 1 um. The electrode is connected through a wire 130 that must be clamped and sealed with an insulating material, e.g. a Teflon sheath.

FIG. 2 shows the agitator, this is made of a carriage frame 150 upon which the agitating blades 160 are mounted as well as a set of 3 electrodes 170. One aspect of the agitator design is maintaining the flatness of the plane on which the blades and electrodes sit 180, the flatness of the plane on which the carriage sits 190 and the relation between the two. Ideally, the complete system is machined on a multi-axis CNC that can keep all these relationships within 1 um. If this is not possible the system has to be machined as separate components and then assembled on a sufficiently flat surface, e.g., a granite slab, using offset blocks engineered to be within 1 um to align the components.

FIG. 3 shows the rail frame 210, 220 and rails 200 upon which the carriage is mounted. The structure is assembled on a flat granite table so that the plane on which the rails run 240 is flat to within 1 um. The rails 200 are kept taut with nuts 250 and on the plane 240 by pre-stressing them in tension on the top cross-bars of the frame 210 whereby the longitudinal cross bars 220 are put in compression. Again the same aspect of the design is considered here—to ensure a flat plain for the carriage 260 to run on.

FIG. 4 shows the complete electroplater assembly. The plating chamber 310 has three micrometers 300 mounted on the back plate 311 of the chamber. The micrometers are isolated from the plating solution using seals 320. The micrometers allow for the adjustment of the chuck 340 so that its front plane 330 can be adjusted in relation to the agitator 380. As the agitator assembly 390 ensures that the agitator blades ride on a plane that is no more than 2 um in undulation it is possible to offer the front face of the chuck up to the agitator with a gap of under 10 um. The anode 420 is isolated from the main plating chamber 310 via a frit 400 and held in its own chamber 410.

FIG. 5 shows the system level set-up of the electroplater. A reciprocating actuator 500 is used to actuate the agitator 570 inside the plating chamber 510. Electrical connection to the working electrode is achieved through the chuck 520. The plating chamber 510 and the anode chamber 530 are both serviced by plating and anode reservoirs 550 and 540 respectively. Flow between the chambers is provided by a pump 560. All chambers 550, 540, 530 and 510 are sealed and contain either an argon blanket or a pressurized inert gas to keep air, and more specifically, oxygen, out of the system.

FIG. 6 shows a variant on the design whereby the chuck 610 is enhanced with a reel-to-reel unit to allow an electrically conducting fabric or flexible electronics substrate 630 to pass over the flat face 690 of the chuck. Electrical connection to the fabric or flexible electronics substrate 630 is made at either the feed or return roller 640 and 650 respectively. The fabric or flexible electronics substrate passes out over an idler roller 660 designed to keep it tight to the chuck face. The bottom of the chuck 611 is designed to be smooth such that the fabric or flexible electronics substrate passes over it without fouling (it could also be a roller). The roller 670 is placed on the back of the chuck to keep the fabric or flexible electronics substrate tight to the chuck prior to it returning to the collector roller 650. The complete system is encased in a feed chamber 630 that allows for either an argon blanket or other inert gas (such as, but not limited to, nitrogen) with a pressure sufficiently above ambient pressure to dispel air, and more specifically oxygen, from the system.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment is for electroplating semiconductor or MEMS wafers. In this embodiment, two planes are established; one being the wafer surface and the other containing the bottom edges of the blades of the agitator. To ensure that the agitator runs on a plane with flatness tolerance of <=2 um, it needs to either be machined out of a single piece of appropriate material (e.g. PEEK) on machinery capable of achieving the desired tolerances, or, it can be assembled on a flat surface with undulations below 1 um. FIG. 2 shows the two main plains that need to be machined to within 1 um of one another; plane 180 is the bottom plane that contains the bottom edges of the agitator blades. This is the plane that is offered up to the wafer during agitation. Plane 190 is the plane defined by the rails on which the agitator runs while being actuated. Hence, the agitator frame 150 can be looked upon as a carriage that runs on rails sitting on the plane 190. For the system to work correctly the rails that the carriage runs on must likewise sit on a plane that undulates less than 1 um. The frame for mounting the carriage rails is shown in FIG. 3. In this embodiment, the rails are a set of parallel rods 200. The rods are pulled taut on end plates 210 by tightening nuts 250 which therefore compress the two exterior frame blocks 220. By mounting the agitator carriage on these rails it is possible to get the desired sub-2 um undulation in the plane on which the bottom of the agitator blades rides.

The electroplating bath is designed so that the agitator assembly sits inside the bath. However, the main frame for tensioning the rails can be engineered to sit outside of the plating solution. FIG. 4 shows an example of the electroplating bath 310 with the agitator 380 and frame 390. A chuck 340 is used to offer the wafer up to the agitator. The wafer is connected to the power supply via a wire 351. Springs are used to keep the chuck in place. By engineering the system so that there is control on the chuck and more specifically the ability to offer the wafer surface 350 up to the plane on which the agitator blades ride, it is possible to bring the moving agitator blades to within less than 5 um of the surface of the wafer.

The offset between the wafer chuck and the plane that the agitator blades sit on can be adjusted by three micrometers 300 that the chuck is mounted on in the plating chamber. This offset can be set by taking a dummy wafer of known thickness 350 and placing a shim 361 of thickness equal to the desired offset (e.g. 5 um) on the face of the wafer. By adjusting the micrometers until the shim no longer fits loosely, the correct offset (e.g. 5 um) can be recorded for each micrometer. Furthermore, after the shim is removed the wafer face can be moved closer or further away from the agitator by adjusting the micrometers. In practice, the gap will most likely need to be maintained constant and adjustments can be used to account for variations or changes in wafer thicknesses.

An alternative method of maintaining offset can be achieved by placing electrodes on the agitator and measuring the resistance of the solution from the electrode to the surface of the wafer (working electrode). FIG. 1 shows an example of an alignment electrode. The core of the electrode 100 is made of a noble metal, e.g., platinum. The electrode is cased in an inert polymer, e.g. PEEK, and the electrode connected to a wire 130 with an inert outer casing such as Teflon. The wire will need to be long enough to allow it to run out of the plating bath and have enough slack on it to allow the agitator to reciprocate. One parameter of the design is that the bottom face of the electrode is machined on a flat plane 120 undulating less than 1 um. In FIG. 2, the electrodes 170 are mounted on the same plane 180 as the bottom of the agitator blades. By knowing the resistivity of the plating solution and the surface area of the electrode it is possible to calculate the offset of the electrode from the surface of the working electrode (wafer). Alternately, if the ‘shim’ method has been used to calibrate the system, the resistivity between electrode and wafer can be noted at a pre-determined distance (e.g. Sum) and used as a set-point to maintain that offset. An advantage of the electrode method is that it allows real-time offset monitoring. Hence, if a certain thickness of material is being deposited (e.g. >10 um), the desired offset can be maintained during plating.

The remainder of the electroplater design in this embodiment builds on well-known and commonly used techniques: Referring back to FIG. 4, the plating chamber 310 has a separate anode chamber 410 to isolate unwanted compounds (created from anode reactions) from entering the plating solution. This is achieved by using a frit 420 between the two chambers. The system level drawing in FIG. 5 shows a method of actuation by the agitator 570 using a reciprocating actuator 500. Both the plating chamber 510 and the anode chamber 530 are serviced with pumps 560. These pumps allow plating solution and anode solution to be replenished from plating 550 and anode 540 reservoirs. All chambers in the system are sealed from the ambient environment to prevent oxygen entering the system. Reservoirs can be bubbled with wet inert gases to also prevent oxidation.

FIG. 6 illustrates an embodiment of the system where the chuck 610 is adapted to feed a roll of fabric or flexible electronics substrate 600 over the flat plane of the chuck that is offered up to the agitator. In this embodiment, parallelness of the plane of the agitation relative to the working electrode (fabric or flexible electronics) is still achieved, even with a flexible material. The conducting fabric or flexible electronics in this case must be pre-manufactured to have a conducting layer on it, such as copper. The cathode connection to the working electrode can be either at the feed spool 640 or the return spool 650. In this embodiment, sufficient tension is maintained on the fabric or flexible electronics material to prevent it from contacting the agitator. This is achieved with tensioners on the chuck at the feed into the chuck 660 and as the fabric or flexible electronics substrate returns 670, additionally, the feeder spool has an amount of friction to ensure tension on the fabric or flexible electronics substrate 600. The system is designed to be air tight using a shroud 630 so that argon or an inert gas can be used to protect the plating solution from oxidation. This allows complex alloys such as Permalloy to be deposited (electroplated) on to the conducting fabric or flexible electronics. 

1. A method of electroplating complex alloys comprising: providing an agitator with an agitation plane of predetermined flatness; providing an agitator carriage oriented parallel to said agitation plane to within a predetermined parallelness; providing a rail system oriented parallel to said agitator carriage to within a predetermined parallelness; providing an electroplating chamber configured to contain said agitator; providing a chuck with a face of predetermined flatness and sprung onto a plurality of actuators with predetermined positioning precision: whereby it is possible to position said face of said chuck relative to said agitation plane to within a predetermined accuracy in parallelness.
 2. A device for electroplating complex alloys comprising: an agitator with an agitation plane of predetermined flatness; an agitator carriage oriented parallel to said agitation plane to within predetermined parallelness; a rail system positioned substantially parallel to said agitator carriage; an electroplating chamber containing said agitator; a chuck with a face of predetermined flatness and sprung onto a plurality of actuators with predetermined positioning accuracy: whereby it is possible to position said face of said chuck relative to said agitator plane to within a predetermined parallelness.
 3. The method of claim 1, wherein the position of said face of said chuck can be held substantially parallel to said agitator plane and within a relative separation of 5 microns.
 4. The method of claim 1, wherein the position of said face of said chuck can be held substantially parallel to said agitator plane and within a relative separation of more than 5 microns and less than 200 microns.
 5. The method of claim 1 further comprising: providing a shim sandwiched between said working electrode and said agitator; adjusting the micrometers on the electroplater while using the shim as a guide; whereby a predetermined offset between the plane of the working electrode and said agitation plane can be set.
 6. The method of claim 1 further comprising: providing a plurality of electrodes with faces machined substantially flat; mounting said electrodes on said agitator such that the faces of said electrodes are substantially parallel to the plane of agitation; measuring the resistance of the plating solution between the electrode and the working electrode; whereby the offset of the working electrode in relation to the plane of the agitator can be determined.
 7. The method of claim 1 wherein said chuck can hold and electrically connect to the wafer; whereby complex alloys can be homogenously deposited on the wafer as the solution on the wafer face is agitated within the Nernst diffusion layer.
 8. A method for electroplating complex alloys on reels of fabric or flexible electronics materials comprising: providing an agitator with an agitation plane of predetermined flatness; providing an agitator carriage oriented parallel to said agitation plane to within a predetermined parallelness; providing a rail system oriented parallel to said agitator carriage to within a predetermined parallelness; providing an electroplating chamber; providing a fabric or flexible electronics substrate with a pre-deposited conducting layer; providing a customized chuck that allows said fabric or flexible electronics substrate to be fed over said customized chuck face so that said fabric is held parallel to the plane of agitation; providing electrical connection to said fabric or flexible electronics substrate such that it becomes the working electrode; whereby complex alloys can be homogenously deposited on said fabric or flexible electronics as the solution on the working electrode face is agitated within the Nernst diffusion layer.
 9. A method of electroplating complex alloys comprising: providing a working electrode; providing an agitator with a plurality of blades; providing a chuck to hold the working electrode such that the relative separation of the edges of said blades from said working electrode is maintained within 5 microns; whereby agitation is achieved within the Nernst diffusion layer of said working electrode.
 10. The method of claim 9, wherein the relative separation of the edges of said blades from said working electrode is maintained between 5 microns and 200 microns; whereby agitation is achieved within the Nernst diffusion layer of said working electrode.
 11. The method of claim 9, wherein the working electrode is fabric or flexible electronics.
 12. The method of claim 9, wherein the complex alloy material is superconducting.
 13. The method of claim 9, wherein the working electrode material is superconducting. 